1. Field of the Invention
The present invention relates to a process improving the dispersibility and the filterability of water-soluble polymers when these polymers are packaged in powder form. This process is applicable to natural (extracted) polymers, to fermentation polymers and especially to scleroglucan, and to synthetic polymers, each time the process of manufacture of the polymer includes a stage of separation by precipitation by means of a nonsolvent.
Water-soluble polymers have found many industrial applications, especially in the petroleum field, in particular for improved recovery of hydrocarbons, for the prevention of the rush of water into a producing well from a water-bearing reservoir, or from flood water and for modification of the injection profile, in the case of which operations the treatment may consist in injecting water-soluble polymers at the level of the strata whose permeability at right angles to the well it is intended to reduce. The present invention is related to the use of polymers having improved properties to enhance oil recovery in this manner.
The water-soluble polymers employed in these various petroleum applications are required to have precise characteristics, two of which are particularly important, namely the viscosity and the filterability of the solutions prepared from these polymers. A high viscosity is indispensable to good control of mobility of the fluid in the formation, and a high viscometric efficiency ensures lower operating costs. Good filterability is essential to avoid blocking the formation into which the solution is injected. Dispersibility is also a characteristic to be taken into account. In fact, a high dispersibility promotes a rapid development of viscosity and restricts the use of mechanical dispersers, in which the shear reduces the yield of the product. This characteristic, which is of value when the treatment is performed on shore becomes of great importance in the case of offshore treatments. Under offshore conditions the storage capacities are, in fact, limited, and good dispersibility avoids long maturing of the solutions and excessive immobilisation of these storage capacities.
Available water-soluble polymers, be they natural polymers, fermentation polymers or synthetic polymers, all have a filterability and often a dispersibility which are inadequate to various degrees. These poor characteristics are frequently inherent in the process of manufacture of the polymer. Some stages of the processes sometimes take place under severe mechanical, thermal or chemical conditions which give rise to polymer aggregation factors, structural damage or impurities. These factors are responsible for aggregates of polymer molecules, called microgels, which, even when present in small quantity, considerably reduce the filterability of the solutions to be injected into the reservoir.
2. Description of the Prior Art
Many processes have been proposed in the prior art to improve the filterability and dispersibility of solutions of water-soluble polymers and thus to obtain a satisfactory injectability of these solutions into the reservoir. In very many cases these processes apply to the water-soluble polymer only once it has been dissolved. Among these processes there may be mentioned the incorporation of surfactants in the polymer solution. French Patent 2,600,336 and U.S. Pat. Nos. 3,853,771, 3,801,502 and 4,212,748, for example, recommend the use of surfactants to improve the injectivity of solutions of water-soluble polymers.
It should be noted, however, that while positive results can be obtained with these various processes, they all present a handicap which rules them out. Since they apply to the polymer once it has been dissolved, their application necessarily takes place on the oil production site, and this involves additional costs and specialised teams to carry out the treatments. As a result, no process for improving the filterability of water-soluble polymers in solution appears to be functioning on an industrial scale at this time.
To get around this difficulty, a number of processes producing a formulation which can be exploited directly have been proposed, some of which produce a fluid composition of the polymer and others directly with the polymer in powder form, which are preferred by the oil producers because of their limited bulk. An improvement in the dispersibility of the powder obtained is claimed, for example, in U.S. Pat. No. 4,654,086 by mixing a surfactant with xanthan powder. The process described in French Patent 2,577,982 shows that the addition of alkali metal lignosulphonate to xanthan powders, scleroglucan and polyacrylamides appreciably improves the dispersibility of these Water-soluble polymers and the filterability of the resulting solutions. It must be noted, however, that water-soluble polymer powders are well known, on the one hand, to have a poor dispersibility which affects the final viscosity of the solutions and, on the other hand, to produce solutions of more mediocre filterability than those from which they are derived. The origin of these poor characteristics of the water-soluble polymer powders is to be found in the process processing from the fermentation broth to the powder formed polymer. This change from a liquid form to a solid form tends to degrade. In the case of fermentation polymers, xanthan and scleroglucan, the solution containing the polysaccharide is mixed with an equivalent or larger volume of an organic nonsolvent, generally isopropanol. This operation, called coagulation, precipitates the polysaccharide in the form of a fibrous mass, still filled with water and with nonsolvent. This precipitate, called coagulum, is then pressed to remove as much as possible of the liquid phase impregnating it, and is then broken into pieces, dried' and ground. A xanthan or scleroglucan powder is collected at the end of these operations. Despite the great precautions which are taken, some deterioration in the quality of the polymer cannot be avoided, due to a partial denaturing of its structure which, if excessive, can go as far as to make the polymer insoluble.
As demonstrated in the prior art and especially in French Patent 2,577,982, the incorporation of surfactants makes it possible to restore to the denatured powders a dispersibility and a filterability which in turn makes it possible to envisage their injection into the reservoir. It has been found, in a completely unexpected manner, that the incorporation of surfactants in the polymer can be advantageously carried out when the polymer is still in the form of coagulum and especially when this polymer is a polysaccharide of the scleroglucan type.